Dragonstone is the castle that stands upon the eponymous island located in Blackwater Bay. It is the ancestral seat of House Targaryen and was the stronghold of a cadet branchof House Baratheon.
At the time the series begins, it is held for King Robert Baratheon by his brother, Lord Stannis. The Chamber of the Painted Table is inside the castle. The castle was built using advanced Valyrian stonemasonry techniques, long since lost. As a result it has many exotic features unique in all of Westeros, including massive dragon-shaped gargoyles. The entire castle is dominated by dragon motifs and reliefs.
Prior to planning her invasion, Daenerys landed on her headquarters on her ancestral home.
(source GOT wiki)